Energy alternatives to conventional energy sources have long been under investigation. Notable among these is solar energy, where considerable achievements have been attained in terms of solar energy collection, but with less than satisfactory results in terms of thermal energy storage.
Substantial efforts have also been expended to utilize conventional energy sources more efficiently and uniformly by storage of energy produced during off-peak periods for subsequent use during peak-use periods. One such proposal involves utilization of electric immersion heaters during off-peak hours to transfer heat to a thermal storage medium for subsequent use in space and water heating. Again, particularly for the individual consumer, off-peak thermal energy storage has remained a major problem.
Thermal energy storage systems in current use include a first category which is reliant upon the specific heat of a storage medium, such as a large body of water or a large mass of rocks or bricks, and a second category which utilizes various chemical storage media that are reliant primarily on the latent heat of fusion of the media. Included within such chemical media are phase change materials and chemical change materials, i.e., materials which undergo physical and/or chemical changes at a given temperature. As is known, the amount of heat required to be added to or removed from a material to effect a physical and/or chemical change (e.g., from solid to liquid) is far greater than that required to merely raise or lower the temperature of the material at any other thermal level. Thus, by relying on the latent heat of fusion, far greater thermal storage capacity can be obtained per unit weight or volume of storage medium. Consequently, chemical storage media, i.e., latent heat of fusion media, offer more potential for the future than specific heat media.
Representative chemical storage media include various salts, salt hydrates, eutectic mixtures, waxes, oils and long chained chemical compounds. A particularly advantageous material for use in domestic, as well as commercial and industrial, thermal energy storage systems is sodium thiosulphate pentahydrate, a relatively common and inexpensive salt hydrate which undergoes a solid/liquid phase change at a temperature of approximately 188 F., with a latent heat of fusion of about 9,360 Btu's per cubic foot. Thus, the material is ideally suited for high density thermal storage for space and water heating, whether the input thereto is thermal energy derived from the sun, off-peak electrical or hydrocarbon energy, or both.
However, the material suffers the disadvantages that it is a poor conductor, heat tends to collect in isolated places, salt hydrates tend to separate or destabilize when in a liquid state thereby mitigating against recycling through the phase change and causing severe loss of heat storage capacity, and so-called "super-cooling", i.e., dropping below the phase change temperature without giving off the previously stored latent heat of fusion.